Electric field tuning of PbS quantum dots for high efficiency solar cell application

ABSTRACT

A thin film and a method of making a thin film. The thin film comprises a patterned substrate, a smooth film of electric field tuned quantum dots solution positioned on the patterned substrate, and a thin layer of metal positioned on the thin film. The method begins by drop-casting a quantum dots solution onto a patterned substrate to create a thin film. While the quantum dots solution is drying, a linearly increasing electric filed is applied. The thin film is then placed in a deposition chamber and a thin layer of metal is deposited onto the thin film. Also included are a method of measuring the photoinduced charge transfer (PCT) rate in a quantum dot nanocomposite film and methods of forming a Shottky barrier on a transparent ITO electrode of a quantum dot film.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to currently pending U.S. ProvisionalPatent Application 61/236,271, filed Aug. 24, 2009, which is hereinincorporated by reference.

FIELD OF INVENTION

This invention relates to solar cells; more specifically to the use ofquantum dot films in solar cells.

BACKGROUND

More energy from the sun strikes the Earth in one hour than all theenergy consumed on the planet in one year, yet solar electricityaccounts for less than 0.02% of all electricity produced worldwide. Theenormous gap between the potential of solar energy and its use is due,in part, to the cost/conversion capacity. The development of thirdgeneration solar cells (high efficiency plus low cost) is of paramountimportance to both humanity and nature.

Solution-processability has been recognized as a feasible solution tocost issues, and novel mechanisms such as carrier multiplication apossible route to achieve higher efficiency levels. In both theseaspects, there is potential in colloidal infrared quantum dots, such aslead selenide (PbSe) and lead sulfide (PbS). The addition of quantumdots to present cost-effective organic solar materials (i.e., polymers)could double power conversion efficiency to twelve percent due to theinfrared absorbers' enhanced spectrum match with sunlight. Initialobservation of carrier multiplication and recent confirmation of it inthese quantum dots holds fundamental importance in current solar celldevelopment.

However, present PbSe and PbS quantum dots research has seemed to hit a‘bottleneck’, hindering the achievement of their full potential.Efficient photo-induced charge transfer has not been observed in thesequantum dot composites, due largely to the lack of a measurementtechnique which would allow a clear separation between excitondissociation and charge transport phenomena. This makes it challengingto gain detailed insight into either phenomenon, impeding rationaldesign of absorber layers. Furthermore, the majority of the transportstudies so far have been limited to the planar structure field effecttransistors (FET), whereas an applicable quantum dot photovoltaic (PV)device is of sandwich structure, and it is known that the transportcharacteristics could be very different in these two structures.

Quantum dots are essentially nanocrystals consisting of tens to hundredsof atoms. FIG. 1A illustrates the rock salt crystal structure of PbSequantum dots. Due to the nanocrystal's small size (smaller than theexciton Bohr radius of the bulk semiconductor), strong quantumconfinement results in discrete energy levels and bigger band gapscompared with the respective bulk semiconductor. FIG. 1B shows thequantized energy levels of a PbSe quantum dot. The dashed linerepresents the gap state energy level found on IV-VI quantum dots.Infrared quantum dots such as PbSe or PbS have size-tunable band gapsranging from 0.4˜1.1 eV. Consequently, their optical absorption coverssolar spectrums from infrared to ultraviolet. The graph of FIG. 1Cillustrates the absorption spectra of variously sized PbSe quantum dots.Arrows indicate the first excitonic peak (1S_(h)−1S_(e)) in the infraredregion. Another tunable factor in quantum dots comes from theirpassivation layer (ligands), which largely influences optical andelectronic properties.

In general, the photovoltaic (PV) process in the quantum dots system(quantum dots with one or two other constituents) consists of foursuccessive processes:

-   -   (i) absorption of photons, which creates excitons (bounded        electron-hole pairs);    -   (ii) exciton dissociation (or photo-induced charge transfer)        following the exciton diffusion to a region (for instance, the        interface of two different components);    -   (iii) free carriers transport separately toward the anode        (holes) and cathode (electrons), where    -   (iv) charge collection occurs.

As an example, FIG. 2A is a diagram of a hybrid PV device made of PbSequantum dots and conducting polymer P3HT. FIGS. 2B and 2C show how photocurrent is generated in the hybrid PV device. FIG. 2B shows electron etransferring from the P3HT polymer to the PbSe quantum dot. FIG. 2Cshows the hole h transfer from the PbSe quantum dot to the P3HT polymer.The hybrid device of FIG. 2A is more thoroughly described in U.S. patentapplication Ser. No. 12/630,398, filed Dec. 3, 2009, which is hereinincorporated by reference.

Despite being one of the most promising solutions for solar energyutilization, the present performance of such infrared quantum dot-basedPV devices are far from their expectations. The two major causes fortheir relatively low efficiency have been recognized in currentresearch:

-   -   (1) inefficient exciton separation at the quantum        dot/constituent interface; and    -   (2) poor charge percolation pathways to the extracting        electrodes

During the colloidal synthesis process, certain ligands (usually TOPO oroleic acid) are used to passivate quantum dot surfaces to preventaggregations. Incomplete passivation could result in surface trap sites,and with the bulky ligands serving as barriers for exciton dissociationat the quantum dot/constituent interface, photo-induced charge transfer(PCT) is hindered. The addition of quantum dots without the optimizationof their interfaces with other constituent(s)—i.e., without theformation of separate percolation pathways for electrons-e andholes-h—causes huge loss of the photo-generated free carriers due to e-hrecombination.

Various kinds of post-synthesis chemical treatments of quantum dots haveproven to be efficient to enhance quantum dot transport propertieswithout sacrificing their confinement uniqueness. Recently, a series ofstudies on quantum dot device physics regarding ligand exchange withbutylamine have demonstrated improved infrared response of PbS quantumdot photovoltaic devices and photoconductors. Thermal treatment isanother method to improve carrier mobility and conductance of PbSe NCfilm via enhanced interdots electronic coupling.

SUMMARY

A thorough transport study in infrared quantum dot systems is vital tothe fabrication of high-performance quantum dot-based solar cells.Current research also desires an effective way to finely tune quantumdot transport properties along the desired current flow direction.

The present invention includes, inter alia, a unique spectral gaugemeasurement and a novel electrical tuning method for directed transportproperties. The combination of such method can be used in thefabrication of commercially viable quantum dot-based solar cells.

The present invention includes a method of making a thin film (using anelectrical tuning method). The method begins by drop-casting a quantumdots solution onto a patterned substrate to create a thin film. Thesubstrate may be a glass substrate. Such a glass substrate may be anindium tin oxide (ITO) glass substrate. The quantum dots solution may bea lead sulfide (PbS) quantum dots solution. While the quantum dotssolution is drying, a linearly increasing electric field is applied. Inan embodiment, the electric field starts at about 5V and linearlyincreases in about 5V increments up to about 50V. The electric filed ispreferably a DC electric field. The duration between each incrementincrease may be between about one and about two minutes. The thin filmis then placed in a deposition chamber and a thin layer of metal isdeposited onto the thin film. There may be a twenty minute wait timeafter applying the electric field before placement of the thin film intothe deposition chamber.

The deposition chamber may be an organic deposition chamber. Thedeposition chamber may be integrated with a N2 glove box. The metaldeposited on the thin film may be aluminum.

The present invention also includes a thin film (made using anelectrical tuning method). The thin film comprises a patternedsubstrate, a smooth film of electric field tuned quantum dots solutionpositioned on the patterned substrate, and a thin layer of metalpositioned on the thin film. The substrate may be a glass substrate.Such a glass substrate may be ITO glass substrate. The quantum dotssolution may be a PbS quantum dots solution. The metal deposited on thethin film may be aluminum.

The present invention further includes a method of measuring thephoto-induced charge transfer (PCT) rate in a quantum dot nanocompositefilm (the spectral gauge measurement). The method begins by measuringthe photo-induced absorption spectra of a plurality of quantum dotnanocomposites. Below-gap excitations and above-gap excitations areconducted in the measurement of the photo-induced absorption spectra.The change of the spectral gauge is monitored in terms of energy leveland intensity. The PCT rate is derived based on the change of thespectral gauge.

In addition, the present invention includes a method of forming aShottky barrier on a transparent ITO electrode of a quantum dot film.The method comprises spin coating a thin layer ofPoly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) ontothe ITO electrode, and then applying reverse bias to an extractioncontact of the quantum dot film.

The present invention also includes another method of forming a Shottkybarrier on a transparent ITO electrode of a quantum dot film. In thismethod, a thin layer of gold is evaporated onto the ITO electrode, andthen a reverse bias is applied to an extraction contact of the quantumdot film.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference should be made tothe following detailed description, taken in connection with theaccompanying drawings, in which:

FIG. 1A is a diagram of the rock salt crystal structure of a leadselenide (PbSe) quantum dots.

FIG. 1B is a diagram of the quantized energy levels of a PbSe quantumdot. The dashed line represents the gap state energy level found onIV-VI quantum dots.

FIG. 1C is a graph of the absorption spectra of variously sized PbSequantum dots (QD). Arrows indicate the first excitonic peak(1S_(h)−1S_(e)) in the infrared region.

FIG. 2A is a diagram of a bulk heterojunction (BHJ) hybrid devicestructure.

FIG. 2B is a diagram of an electron-e transfer from P3HT polymer to PbSequantum dot

FIG. 2C is a diagram of a hole-h transfer from PbSe quantum dot to P3HTpolymer.

FIG. 3A is a graph of the photo-induced absorption (PA) spectra of a 4.2nm PbS quantum dot film measured at T=10K. The inset (right) shows aschematic diagram of the relevant transitions and position of the bandgap state level. E₁ and E₂ are the first and second interbandtransitions, respectively. E₁ is also the quantum dot optical band gap(E_(g)).

FIG. 3B is a diagram of Auger Recombination showing loss of multipleexcitons by non-radiative recombination.

FIG. 3C is a diagram of Radiative Recombination showing loss ofexcitons.

FIG. 3D is a diagram of Gap State recombination showing loss ofexcitons.

FIGS. 4A-4C are diagrams of IR-PA as a gauge measure of PCT in quantumdot composites.

FIG. 4A illustrates an electron transfer from polymer to quantum dot.

FIG. 4B illustrates an electron relaxing to the gap state.

FIG. 4C illustrates the enhanced IR-PA signal due to increase ofpopulation at the gap state.

FIG. 5A is a diagram of a PbS quantum dot without (left) and with(right) electrically aligned oleic acid ligands;

FIG. 5B is a diagram of quantum dots in an untreated film (left) and inan electrically treated film (right). The quantum dots in theelectrically treated film (right) are closer packed than the quantumdots in the untreated film (left). The quantum dots of the electricallytreated film (right) have a shorter vertical transport path and arebetter electronically coupled than the quantum dots of the untreatedfilm (left)

FIG. 5C is a graph comparing the photocurrent verses voltage of aelectrically treated thin film and an untreated thin film. Thecomparison shows over six order improvement of photocurrent underillumination from an AM 1.5 solar simulator.

FIG. 6 is a graph illustrating the absorption spectrum of 9 nm PbSequantum dots in diluted tetrachloroethylene solution. The inset shows ahigh resolution TEM image demonstrating the monodispersity of theprepared quantum dots.

FIG. 7 is a diagram illustrating continuous wave photo-inducedabsorption spectroscopy. A regular absorption spectrum of a quantum dotis modified by pump-populating a gap state. This gives rise to a newabsorption peak ΔE, whose intensity is directly proportional to theelectron density on the gap state.

FIG. 8A is a diagram illustrating energy level positions of exemplarymaterials measured by electrochemical cyclic voltammetry.

FIG. 8B is a diagram of the chemical structure of novel polymers HT-HTPDDTV (left) and TT-HH PDDTV (right)

FIG. 8C is a diagram of the chemical structure of a novel polymer.

FIG. 9A is a diagram illustrating the possible energy transfer (ET) andphoto-induced charge transfer (PCT) in a polymer/QD system, under typeII band alignment. The solid curved arrow stands for electron transferto QD, whereas the empty curved arrow stands for hole transfer topolymer. The straight empty arrow indicates energy transfer.

FIG. 9B is a diagram illustrating the possible ET and PCT in apolymer/QD system, under type I band alignment. The straight empty arrowindicates energy transfer.

FIG. 10 is a diagram of the setups for time of flight (TOF) and ChargeExtraction by Linearly Increasing Voltage (CELIV).

FIG. 11 is a photograph of customized Mbraun MB200M glove box system,integrated with

ngstrm

mod four-source co-deposition system for high vacuum (<10⁻⁷ torr) metaland organic deposition.

FIG. 12A is a flowchart illustrating the fabrication process for asandwich photovoltaic (PV) device.

FIG. 12B is a flowchart illustrating the fabrication process for aplanar FET device.

FIG. 13A is a diagram of a cross sectional view of a PV devices. In thesandwich structure device, the vertical transport properties of treatedquantum dots can be studied.

FIG. 13 B is a diagram of a cross sectional view of a FET device. In theFET device, the difference in horizontal transport properties of quantumdots with electric fields applied parallel (upper right) andperpendicular (lower right) to the gap can be studied. With gatemodulation it is then possible to obtain density of states forelectrons.

DETAILED DESCRIPTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings, which form a parthereof, and within which are shown by way of illustration specificembodiments by which the invention may be practiced. It is to beunderstood that other embodiments may be utilized and structural changesmay be made without departing from the scope of the invention.

Recent chemical and thermal treatment methods, although showing successin improving quantum dot carrier mobility and film morphology, lack theability to differentiate quantum dot transport properties alongdifferent directions. A novel method of achieving free carrier transportof quantum dots in a more controlled way along the desired transportdirection is described herein.

Spectroscopic Gauge to Measure Photo-Induced Charge Transfer (PCT)

A gap state was discovered in pristine lead sulfide (PbS) quantum dotfilms that shows a confinement-dependent absorption (IR-PA) in the nearinfrared range. FIG. 3A shows the photo-induced absorption (PA) spectraof a 4.2 nm PbS quantum dot film measured at T=10K. The inset shows aschematic diagram of the relevant transitions and position of the bandgap state level. E₁ and E₂ are the first and second interbandtransitions, respectively. E₁ is also the quantum dot optical band gap(E_(g)).

By measuring the frequency dependence of this IR-PA, the lifetime ofthis gap state was estimated to be around several microseconds. Theimportance of such gap state is illustrated in FIGS. 3B-3D through theanalysis of exciton loss mechanism in quantum dots.

As can be seen in FIG. 3B, Auger Recombination occurs within sub-ps,followed by hot exciton cooling within ps scale. The RadiativeRecombination, illustrated in FIG. 3C, was surprisingly slow in thesequantum dots in the range of sub-μs, whereas the relaxation to the gapstate happens much faster (<ns). Therefore, the final state ofphoto-generated carriers is the gap state. Due to its long lifetime,this state is directly related to quantum dot device efficiency. Thisgap state could be used to monitor photo-induced charge transfer (PCT)between quantum dots and polymers, similar to the case seen inπ-conjugated polymer and fullerene systems. Even better, in the hybridnanocomposite of polymers and infrared quantum dots, the detection ofthe PCT process can be done from PA measurements of both constituents.This provides a more reliable and accurate study of PCT in the hybridnanocomposite. It is especially useful when energy transfer (ET) isinterplayed with PCT.

FIG. 4A through 4C show further why this gap state can be used as aspectral gauge to obtain quantitative information about PCT in quantumdot composites. Given the quantum dot/polymer as an example, when PCToccurs between quantum dots and polymers (with type II band alignment,i.e., an electron transfers from polymer to quantum dots), the increasedpopulation at the gap state will result in an increase of the IR-PAsignal. Furthermore, the lifetime of the gap state will be altered dueto the population change. Both differences can be measured by continuouswave photo-induced absorption spectroscopy. On the other hand, if thereis no PCT, no change of IR-PA of quantum dots will be seen.

Directed Carrier Transport in Quantum Dots Layer The as-synthesizedquantum dots are practically insulators due to the original bulkypassivation ligands. For application in any optoelectronic device,quantum dots have to be treated chemically and/or thermally to becomeelectronically coupled. Although such treatments have remarkably boostedquantum dot conductivity and mobility, the controllability of transportmobility is quite low. Quantum dot photovoltaic devices are normally ofa sandwich structure as shown in FIG. 5B, and vertical carrier transportis of more importance than that the current flow parallel to thesubstrate direction. It is thus important to be able to further ‘guide’the carrier transport to have certain preference of direction, i.e.,anisotropic transport.

Because the original ligand of quantum dots, the oleic acid, is polar, aDC electric field can be used to ‘align’ the ligands with the externalelectric field so the transport perpendicular to the electric field hasfewer barriers. Over four orders improvement of photoconductivity atzero bias and more than six orders improvement at 5V bias have beendemonstrated in a sandwich structure quantum dot photovoltaic cell.

In an embodiment, the thin film made using an electrical tuning methodcomprises a patterned substrate, a smooth film of electric field tunedquantum dots solution positioned on the patterned substrate, and a thinlayer of metal positioned on the thin film. The substrate may be a glasssubstrate. Such a glass substrate may be ITO glass substrate. Thequantum dots solution may be a PbS quantum dots solution. The metaldeposited on the thin film may be aluminum.

In an embodiment, the method of making the thin film using theelectrical tuning method begins by drop-casting a PbS quantum dotssolution onto a patterned indium tin oxide (ITO) glass substrate. Duringthe drying process of the solution, a linearly increasing DC electricfield is applied. In an embodiment, the electric field starts at about5V and linearly increases in about 5V increments up to about 50V. Theduration between each increment increase may be between about one andabout two minutes. The observed DC current was about 35 mA. After abouttwenty minutes, the quantum dots film completely dried up and a smoothfilm was observed (much smoother than the original drop-cast quantumdots from the same solution. FIG. 5B shows the alignment of quantum dotsligands with the electric field and the formation of a much denser filmdue to closer packing of the quantum dots. The thin film is then placedin an organic deposition chamber inside a N2 glove box. A thin layer ofaluminum (e.g. 100 nm) is deposited onto the thin film as a cathode ofthe PV device. The device was measured under AM 1.5 solar simulatedsunlight. Over six order improvement in photocurrent was observed, asshown in the graph of FIG. 5C.

This novel approach not only has the potential to increase quantum dottransport properties, it also could direct carrier transport in adesired way for quantum dot device performance maximization. A thinsmooth continuous quantum dot film is very difficult to achieve, butthrough this novel method, a smoother film has been achieved via closerpacked quantum dots. The novel electrical tuning method not onlysignificantly improves photoconductivity across the device, but alsohelps with quantum dot film morphology which will further enhance chargetransfer between quantum dots and its matrices.

Synthesis and Characterization of Size and Ligand-Controlled PbSe andPbS Quantum Dots

The quantum dots (e.g. lead selenide (PbSe) and PbS quantum dots) withdifferent-sized capping ligands may be synthesized using a modifiedcolloidal synthesis procedure. As an example, FIG. 6 shows an absorptionspectrum of 9 nm PbSe quantum dots synthesized by a modified colloidalsynthesis procedure. The inset shows a high resolution transmissionelectron micrograph (HRTEM) demonstrating uniform size distribution,which is confirmed by the narrow FWHM of the absorption peak at about0.65 eV.

A colloidal synthesis setup can be used to prepare tailored quantumdots. All initial optical characterization including absorption andphotoluminescence can be done using the continuous wave absorptionspectroscopy setup. Crystallinity can be assessed using x-raydiffraction. Size distribution and crystal shape can be characterizedusing transmission electron microscopy. Transport properties such asmobility and conductivity can be measured in either lateral (fieldeffect transistor geometry) or sandwich structures. Temperaturedependence of thin film conductivity can be measured in a Janis 10Kclosed-cycle refrigerator cryostat.

Electronic structure, stoichiometry, and interface chemistry of theprepared quantum dots can also be examined with photoemission. Theseexperiments will yield energy level positions and the surface chemistryof PbSe/PbS quantum dots.

Measuring Photo-Induced Charge Transfer (PCT) in Quantum Dot Compositesusing a Spectral Gauge

Continuous Wave Photo-Induced Absorption (cw-PA) Spectroscopy can beused to study the PCT between quantum dots and its constituents. Bymonitoring the change of IR-PA peak in terms of energy level andintensity, the PCT rate can be derived. The tuning of the PCT rate canbe done via interfacial engineering through various material systems,different passivation layers for quantum dots, and thermal/electricalnanomorphology modification. The considerations may be based on energylevel alignment, dielectric matching, and interfaces.

Although transient absorption spectroscopy is usually considered apowerful technique for studying PCT in nanocomposite film, it isincapable of supplying information regarding long-lived photo-generatedcarriers which are more relevant to device applications. Also, it is anexpensive setup not available in many labs.

Instead, cw-PA spectroscopy has proven to be a convenient and successfultechnique to study any below gap long-lived photo excitations. This hasbeen demonstrated in several amorphous semiconductor systems includingconjugated polymers. FIG. 7 shows the technique schematically, using aPbS quantum dot film as an example probed sample. In principle, thetechnique is a standard absorption measurement combined with a choppedpump laser. The pump (cw Ar⁺ laser) excites the quantum dots withphotons of energy larger than the optical gap of the quantum dots (forexample, E_(g)=1.07 eV for a 4 nm PbS quantum dots). The excitedelectrons thermalize into a long lived gap state, characteristic forIV-VI quantum dots. This changes the absorption spectrum, because nowthe transition ΔE becomes possible. A new peak arises in the spectrum ata wavelength commensurable with ΔE (for example ΔE=0.33 eV for a 4 nmPbS quantum dot), as is schematically indicated in FIG. 7. A feature ofthis measurement is that the magnitude of this absorption peak islinearly proportional to the density of the electrons occupying the gapstate, whereas its energy position indicates the gap state level. Inaddition, cw-PA can measure other characteristics of the gap state,including its lifetime (t) by varying modulation frequency (f), itsactivation energy (E_(T)) by varying sample temperature (T) and itsrecombination kinetics by varying pump light intensity. The versatilityof this spectroscopy is that the absorption and photoluminescence (PL)measurements may also carried out using the same setup.

The photo-induced absorption (PA) spectra of the quantum dotnanocomposites and their constituents can be measured. Both below-gapand above-gap excitations in the PA measurements can be conducted. Bymonitoring the change of the spectral gauge (IR-PA) in terms of energylevel and intensity, the PCT rate can be derived. Through systematicstudies of frequency, temperature and intensity dependences, informationabout exciton recombination kinetics can be obtained.

Photoluminescence (PL) decay can be measured with an ns transientspectroscopy. This will give complimentary information about excitondissociation in the early stage.

PL quenching can be measured in each nanocomposite by measuring the PLquantum efficiency (PLQE) using a special integration sphere. The changeof PLQE can be compared with PA results to further determine theinfluence of interfacial environment on the PCT rate.

Photoluminescence excitation spectroscopy can also be performed on thesame cw-PA setup, with an appropriate narrow band pass filter fixed ateach emission peak of interest. This measurement can help distinguishorigins and correlations of the emission bands. It also can identifypossible energy transfer (ET) between different energy states involved.

Exemplary materials that may be used include IV-VI colloidal quantumdots such as PbSe and PbS. The general concept of nanocomposite is tochoose another constituent material to form certain donor/acceptor (D/A)combinations with these infrared quantum dots to facilitate PCT andsequential FCT processes. FIG. 8A lists the band alignment for a few ofexemplary materials. By careful choice of the material combinations, theband alignment can be tuned from type I (‘straddling’, against PCT) totype II (‘staggered’, in favor of PCT). The exemplary first choice ofconstituent can be another type of colloidal quantum dot such as cadmiumselenide (CdSe) and cadmium sulfide (CdS). These quantum dots havediameters from 2 nm-8.5 nm with tunable band gaps (E_(g)) from 2.8eV˜1.9 eV for CdSe, and 2 nm˜6 nm for CdS (E_(g) from 3.55 eV˜2.65 eV).

The exemplary second choice of constituent material can be several kindsof conducting polymers. For example, Polymers with different band gapssuch as P3HT (E_(g)=1.9 eV), MEH-PPV (E_(g)=2.2 eV), PPE (E_(g)=2.6 eV),and low energy (between 1-2 eV) gap and self-assembly enabled polymers.

For enhanced PCT between polymers and quantum dots, it is important toincrease the miscibility among them and enable the formation of goodinterfaces. Examples of some of these new polymers are shown in FIGS. 8Band 8C.

These two classes of constituent materials differ from each other interms of several material parameters. For instance, the dielectricconstant ∈. The majority of polymers have ∈-2, yet colloidal quantumdots have much bigger ∈ and ∈ greater than 100 has been reported in a 12nm PbSe quantum dot. Dielectric screening of the Coulomb interactionplays an important role in electronic properties of quantum dots. Whenthe high-∈ infrared quantum dots are mixed with low-∈ polymers, thesignificant difference between inorganic dipoles and organic dipoles areexpected to affect the energy and charge transfer in their composites.By choosing two different types of constituents to mix with the infraredquantum dots, the role of dielectric constants could be found tomismatch in these quantum dot composites in PCT. Note that many D/Acombinations could be made through proper choices of different donor andacceptor materials.

Separate Photoinduced Charge Transfer (PCT) and Energy Transfer (ET)

Depending on the band alignment of the constituent and infrared quantumdots, there could be a strong ET process that competes with the PCTprocess. FIG. 9 shows the possible ET and PCT in a polymer/quantum dotsystem. In type I band alignment (‘straddling’ conduction and valencelevels, FIG. 9B), the energetics are dominated by ET, thus one wouldexpect the PL quenching (decrease of PL quantum efficiency) on thevisible constituent (e.g., polymer), yet no change of IR-PA of quantumdots from PA measurement (see FIG. 3B through 3D).

On the other hand, PCT is more favorable when the band alignment betweeninfrared quantum dots and the constituent materials is type-II(‘staggered’ conduction and valance levels, FIG. 9A), and the band edgeoffset is bigger than the exciton binding energy. In this case, the PLquenching will be a combined effect of ET and PCT if the excitationenergy is greater than the bandgap of the visible constituent. Becausethe PCT rate could be measured separately using the spectral gauge, thecontribution from ET would also be separately evaluated. Note that withthe excitation of only the infrared quantum dot, or both constituents,the analysis of ET and PCT can give us additional information regardingelectron transfer and hole transfer, respectively.

Interfaces are very important regarding exciton dissociation and chargetransfer in quantum dot composites. There are several interfaces inquantum dot film: the interface of the ligand/quantum dot surface;interfaces between quantum dots, and interfaces of a quantum dot withsubstrate. These interfaces could serve as traps and recombinationcenters for photo-generated carriers, resulting in loss of collectablecharges and altered lifetime of the gap states. Another complicationcomes from the possible formation of a dipole layer at the interface,which would change the effective band alignment and the quantum dotenergy levels. All these interfacial issues will manifest themselves inthe change of PCT characteristics. Interface engineering can be done viachemical treatments such as ligand exchange in a solution prior to filmformation and ligand condensation or removal on films.

Film morphology plays an essential role in determining optoelectronicproperties and would affect the result of the PCT process. Manipulationof film morphology during and after the film formation process isanother important part of the interface engineering in quantum dotcomposites. The films used for various measurements are made fromsolution (either pure IR NC solution or mixture solution with anothercomponent) by drop-casting or spin-coating. A reliable protocol to makesmooth film on various kinds of substrates (glass, sapphire or Si) canbe fulfilled by a combination of these methods: appropriate cleaningprocedures for different substrates, pre-treatment of substrates viaHMDS vapor, PEDOT:PSS thin layer (<30 nm) and oxygen plasma cleaning. Inaddition, thermal treatments are designed to further enhance filmmorphology. Thermal treatment conditions are expected to vary with otherparameters such as solvent used and concentration of materials.

Measuring Free Carrier Transport (FCT) via TOF and CELIV

Two complementary transient transport techniques, namely the Time ofFlight (TOF) and Charge Extraction by Linearly Increasing Voltage(CELIV), are suitable for measuring FCT because the charge mobility,recombination mechanism, interplay of trap states, and Gaussian DOStransport sites can be simultaneously studied. TOF is limited to lowconductive materials and thick films (d˜μm), whereas CELIV measurementis capable of measuring higher conductivity materials and in muchthinner film (˜100 nm). As shown in FIG. 10, the setups for TOF andCELIV share common features. The second harmonic (523.5 nm) of a Nd:YLFQ-switched infrared ns pulse/CW laser (JDSU M110-1047 series, pulseduration <7 ns) and a Stanford Research Systems (SRS) N₂ laser (337 nm)with a pulse duration of 3.5 ns are used for carrier photo-generation. ASRS DS345 arbitrary pulse generator is used to generate the voltagepulse. A sandwich structure device with appropriate thickness isemployed in both experiments. Either from the transparent ITO or themetal contact side, the pulse laser illuminates, depending on the typeof carriers being extracted.

Time of Flight (TOF)

Time of Flight (TOF) measurement is a basic method to study transportproperties of thin films made with low mobility semiconductors (μ<<1cm²/V·S), such as amorphous silicon and π-conjugated polymers (PCPs).Photo-generated charges are produced near one electrode by a short pulselaser, and then drift through a thick film (˜several μm) toward anotherelectrode under an applied square electric field E (or bias V). From themeasurement of the carrier transit time t_(tr), the mobility (μ) ofcorrespondent carriers (electron or holes) could be calculated as

${\mu = {\frac{d}{\left( {T_{tr}E} \right)} = \frac{d^{2}}{\left( {t_{n}V} \right)}}},$where d is the thickness of thin film. Normally one works in theso-called differential-mode where the time constant of the measurementset-up is shorter than the transit time: τ_(RC)<t_(tr), where currenttransient j(t) is thus measured and t_(tr) is the point where thecurrent transient begins to drop from its plateau region. TOF,alternatively, requires low conductivity to avoid overestimate of samplemobility due to dielectric relaxation, and much shorter chargerelaxation time than the transit time for accurate results.Charge Extraction by Linearly Increasing Voltage (CELIV)

Instead of a constant bias (square) voltage in TOF, CELIV applies alinearly increasing (ramp) voltage (U=At, where A is the voltage risespeed) across a sandwich type sample with two blocking contacts, where atransient current is recorded in the oscilloscope. In a CELIVmeasurement, a short laser pulse is used to generate the chargecarriers, and after a certain delay-time t_(del), the carriers left inthe sample are extracted. The time taken to reach the extraction currentmaximum (t_(max)) is used to estimate the drift mobility of the(photo-generated) charge carriers. Dispersion in CELIV currenttransients can be characterized by the ratio between the time theextraction current reaches its half-width and the time of maximumcurrent,

$\frac{t_{1/2}}{t_{\max}}.$A theoretical calculated value of 1.2 is considered non-dispersivetransport.

Both the hole and electron hole mobility in a pristine PbSe or PbSquantum dot film can be measured using the TOF method because theas-synthesized quantum dots have very low conductivity and mobility. Byvarying the intensity of the incident light pulse, electric fieldstrength, and the sample temperature, the Langevin recombination processand its mechanism can be probed.

Treated quantum dots are expected to have higher conductivity andmobility; the hole and electron hole mobility can be measured using bothTOF and CELIV methods. The comparison between two measurements would beinsightful as to what extent carrier diffusion affects charge transport.By varying sample temperature, we could also determine whether thedispersive transport behavior dominates as previously observed withorganic polymers.

For each quantum dot composite film, both measurements can again beemployed. The focus will be on modified carrier transport due topossible photo-induced charge transfer, comparing with the pure quantumdot film. The bimolecular recombination coefficient (b) and the Langevinrecombination coefficient (β_(L)) may be measured. The ratio b/β_(L)gives a measure to the limiting factor for quantum dot-based PV devices.

Fabrication of quantum dot Devices for TOF and CELIV Measurements

Primarily, devices used in TOF and CELIV measurements are of thesandwich structure (normal PV device). Complimentary planar structure(FET device) can also be fabricated. Devices can be fabricated in anoxygen and humidity controlled device fabrication facility, whichincludes an customized Mbraun MB200M glove box system, integrated with

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mod four-source co-deposition system for high vacuum (<10⁻⁷ torr) metaland organic deposition, as shown in the photograph of FIG. 11. Abuilt-in Laurel programmable spin-coater allows the fabrication ofdevices inside the inert gas atmosphere. This setup provides an idealcontrolled environment for device stability studies. Flow charts of thefabrication processes for the sandwich PV device and planar FET device,are shown in FIGS. 12A and 12B respectively. The fabrication process forthe sandwich PV device is further described in U.S. patent applicationSer. No. 12/630,398, filed Dec. 3, 2009.

Electrically Tuning the Transport in Quantum Dot Film

Significant improvement of photoconductivity across the device has beendemonstrated after treating the pristine quantum dot (with oleic acidligand) film with an electric field.

Electrical treatment can be applied to ligand-exchanged quantum dots.The choice of ligands may be based on polarity. For instance, bothpyridine and butylamine are polar like oleic acid, except much smaller.From this one would expect to see even higher photoconductivity andmobility on these ligand-exchanged quantum dots following the electricfield treatment.

In order to investigate the anisotropic transport properties tuned byelectric fields, the field effect transistor (FET) can be employed formobility measurements, accompanied by a sandwich structure device usedin TOF and CELIV measurements. FIGS. 13A and 13B show a cross sectionview of these two types of devices. In the sandwich structure device(normal PV device), shown in FIG. 13A, one could study the verticaltransport properties of treated quantum dots. In FET device, shown inFIG. 13B, one could study the difference in horizontal transportproperties of quantum dots with electric fields applied parallel (upperright part of FIG. 13B) and perpendicular to the gap (lower right partof FIG. 13B); with gate modulation it is then possible to obtain densityof states for electrons. Combinations from both measurements would forma comprehensive picture about 3-D anisotropy transport characteristicstuned by electric field. Quantum dots with different sizes, ligandgroups and crystalline structures (i.e., rock salt crystal quantum dotlike PbSe, or Wurtzite crystal quantum dots like CdSe) can be chosen.Temperature dependence of conductivity can be measured to withdrawinformation on the transport mechanism.

Important Points

Modeling of Transport Behaviors in Quantum Dots

Colloidal quantum dots bear lots of similarity to polymers in terms ofdiscrete energy levels, tightly bonded excitons, and optoelectronicproperties inherent to film nanomorphology. Hypothetically, the primarytransport theory and models for polymers would be a good start forquantum dot systems.

In low mobility materials, the Langevin bimolecular recombinationdominates and sets an upper limit for extraction of photo-generatedcarriers, thus limiting the efficiency of related PV devices. TheLangevin process is a second order type process. The recombinationcoefficient (β_(L)) is directly proportional to the charge carriermobility

$\beta_{L} = {\frac{e\left( {\mu_{n} + \mu_{p}} \right)}{{ɛɛ}_{0}}.}$Combined with experimental results, a valid model about transportbehaviors in pristine quantum dots and quantum dot composites can bebuilt. This will serve as a guide for rationale design of quantumdot-based solar cells.Formation of the Schottky Barrier

The establishment of the Schottky barrier between the quantum dotsand/or quantum dots with contacts is very important for both TOF andCELIV measurements, because a built-in electric field within thedepletion layer will enhance the extracted current. The premise for anelectron Schottky barrier formation is the existence of a considerabledifference between the electrode work function φ_(m) and the electronaffinity of the semiconductors. So far, Schottky barriers have beenfound to form only at the metal contact side. It remains a challenge toform Schottky barriers with the transparent ITO electrode.

To solve this challenge, the work function of ITO can be modified viaformation of a surface states layer between ITO and the composite. Thiscan be done via spin coating a thin layer of PEDOT:PSS, or evaporating avery thin layer of gold (<20 nm) onto the ITO contact. In addition, anappropriate block contact should be used to avoid injection current.This can be achieved by applying reverse bias to the current extractioncontact.

Trap States Modified Transport Behaviors

Quantum dots are known to have issues with trap states due to incompletepassivation of quantum dots during synthesis. Additional traps statesalso occur during chemical/thermal treatments, and oxidization ofdangling bonds to form lead oxide (PbO), lead sulfite (PbSO₃) and leadsulfate (PbSO₄). Trap states inevitably will affect quantum dottransport characteristics. Various surface passivation approaches can beinvestigated, including selective, additional or reduced passivation ofsurface atoms, to gain control over the surface trap density. Selectivepassivation can be done using oleic acids and tri-n-octylphosphine(TOPO). Previous studies have shown different binding sites for thesetwo ligands on either lead (Pb) or sulfer (S). Additional passivationcan be done via soaking of the original quantum dots in different ligandsolutions. Reduced passivation can be achieved through washingprocedures.

Oxidization affects quantum dot device stability and lifetime. Theintentional oxidization effect can be investigated by exposing quantumdots in pure O₂, O₂+H₂O and O₂+H₂O+light environments. Temperature andO₂ exposure time (in solution or on film) will be some of the controlledfactors. By tuning the trap states via the chemical and/or thermaltreatments and also correlating with the results from cw-PAspectroscopy, the factor that affects mobility the most, interdotsdistance, carrier density, or trap states, will be found.

It will be seen that the advantages set forth above, and those madeapparent from the foregoing description, are efficiently attained andsince certain changes may be made in the above construction withoutdeparting from the scope of the invention, it is intended that allmatters contained in the foregoing description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall there between.

What is claimed is:
 1. A method of making a thin film, the methodcomprising: providing a patterned substrate; drop-casting a quantum dotssolution onto the patterned substrate to create a thin film; while thequantum dots solution is drying, applying an electric field starting atabout 5V and linearly increasing the electric field in about 5Vincrements up to about 50V; placing the thin film in an depositionchamber; and depositing a thin layer of metal onto the thin film.
 2. Themethod of claim 1, wherein the substrate is a glass substrate.
 3. Themethod of claim 2, wherein the glass substrate is an ITO glasssubstrate.
 4. The method of claim 1, wherein the quantum dots solutionis a PbS quantum dots solution.
 5. The method of claim 1, wherein theelectric field is a DC electric field.
 6. The method of claim 1, whereinthe duration between each increment increase in the application of theelectric field is between about one and about two minutes.
 7. The methodof claim 1, wherein the deposition chamber is an organic depositionchamber.
 8. The method of claim 1, wherein the deposition chamber isintegrated with a N2 glove box.
 9. The method of claim 1, wherein themetal is aluminum.
 10. The method of claim 1, further comprising:waiting about twenty minutes after applying the electric field to placethe thin film in the deposition chamber.